Lambda oxygen sensors are used in the exhaust system of internal combustion engines to optimize pollutant emissions and the exhaust-gas aftertreatment. The lambda oxygen sensors determine the oxygen content of the exhaust gas which is then used for the closed-loop control of the air-fuel mixture supplied to the internal combustion engine and thus the exhaust-gas lambda number upstream of a catalytic converter. A lambda control loop controls the supplying of air and fuel to the internal combustion engine in closed loop to achieve an exhaust gas composition that is optimal for the exhaust-gas aftertreatment by the catalytic converters provided in the exhaust tract of the internal combustion engine. In the case of spark ignition engines, a lambda of 1, thus a stoichiometric ratio of air to fuel is typically controlled in closed loop. The pollutant emissions of the internal combustion engine can thus be minimized.
Various forms of lambda oxygen sensors are in use. In the case of a two-step lambda oxygen sensor, also referred to as a discrete-level sensor or Nernst sensor, the voltage-lambda characteristic curve exhibits a step change in the characteristic curve profile at lambda=1. Therefore, it essentially allows a distinction to be made between rich exhaust gas (λ<1) during internal combustion engine operation characterized by excess fuel and lean exhaust gas (λ>1) during operation characterized by excess air, and permits a closed-loop control of the exhaust gas to a lambda of 1.
Using a broadband lambda oxygen sensor, also referred to as a continuous or linear lambda oxygen sensor, the lambda value in the exhaust gas can be measured within a broad range around lambda=1. Thus, for example, an internal combustion engine can also be controlled in closed loop toward a lean operation characterized by excess air.
By linearizing the sensor characteristic, a continuous closed-loop lambda control upstream of the catalytic converter is possible within a limited lambda range even when a less expensive, two-step lambda oxygen sensor is used. This requires that there be a unique relationship between the sensor voltage of the two-step lambda oxygen sensor and lambda. This relationship must exist for the entire service life of the two-step lambda oxygen sensor since, otherwise, the accuracy of the closed-loop control will not suffice, and unacceptably high emissions can occur. This requirement is not met due to manufacturing tolerances and the aging effects of the two-step lambda oxygen sensor. For that reason, two-step lambda oxygen sensors upstream of the catalytic converter are mostly used in the context of a two-step closed-loop control. This has the disadvantage that, in operating modes, for which a lean or rich air-fuel mixture is required, for example, for catalytic converter diagnostics or for component protection, the target lambda can only be precontrolled, but not controlled in closed loop.
It is believed to be understood that there are various methods for calibrating the voltage-lambda characteristic curve of two-step lambda oxygen sensors to ensure that they can be used for a continuous control over the entire operational life thereof.
German Patent Application DE 3827978 discusses determining and compensating for a voltage offset, which is constant over the entire lambda range, of the voltage-lambda characteristic curve in question using a reference-voltage lambda characteristic curve of the two-step lambda oxygen sensor as a basis for comparison by adjusting the sensor voltage upon trailing throttle fuel cutoff of the internal combustion engine. Furthermore, German Patent Application DE 102010027984 A1 describes a method for operating an exhaust system of an internal combustion engine where at least one parameter of the exhaust gas flowing in an exhaust tract is measured by an exhaust-gas sensor. In accordance therewith, fresh air is supplied to the exhaust tract upstream of the exhaust-gas sensor via a fresh air supply assigned to the exhaust system during one operating state of the internal combustion engine in which injection and fuel combustion do not take place, and the exhaust-gas sensor is adjusted during this time and/or subsequently thereto.
However, the voltage offset can only be adequately compensated when it is equally pronounced, not only in the case of trailing throttle fuel cutoff given corresponding oxygen-containing exhaust gas, but also over the entire lambda range. This can be the case when the voltage offset is due to a single cause. For the most part, however, there are several overlapping reasons why the voltage-lambda characteristic curve deviates from a reference-voltage lambda characteristic curve. These may be more or less salient in different lambda ranges, whereby the voltage offset changes as a function of the exhaust gas lambda. In particular, the causes in the lean and rich lambda ranges can vary in saliency. In the case of trailing throttle fuel cutoff, such a lambda-dependent voltage offset cannot be adequately compensated by an adjustment. A further drawback of the method resides in that present-day engine designs feature fewer and fewer trailing-throttle phases, thereby limiting the possibility of such trailing-throttle adjustments.
German publication DE3837984 discusses a method for compensating for a shift of the lambda-1 step of the voltage-lambda characteristic curve by a setpoint control that includes a second lambda oxygen sensor disposed downstream.
German publication DE19860463 discusses a method for determining the composition of the fuel-air mixture of a combustion engine during operation at a given setpoint value deviation from lambda=1, where the actual value deviation from lambda=1 is determined by temporarily adjusting the composition and evaluating the resulting reaction of a lambda oxygen sensor. It provides for a step-type adjustment by a defined value toward lambda=1 to be initially made, and for the lambda value to be subsequently further modified at a defined rate of change until the lambda oxygen sensor reacts, and for the actual deviation to be determined from the value of the step-type adjustment, the rate of change, and the time until the reaction of the lambda oxygen sensor is ascertained.
Using the method, an offset of the voltage-lambda characteristic curve of a two-step lambda oxygen sensor can be recognized. It is disadvantageous that differences in various lambda ranges remain unconsidered in determining the actual value deviation from lambda=1. This can falsify the result to such an extent that the accuracy required for a continuous closed-loop lambda control using a two-step lambda oxygen sensor disposed upstream of the catalytic converter in order to recognize a characteristic curve offset, is not met.